The present invention relates to mass spectrometers.
A mass spectrometer analyzes mass-to-charge ratios of ions, such as charged atoms and molecules, and typically includes an ion source, one or more mass analyzers and one or more detectors. In the ion source, particles are ionized and extracted from a sample. The ions are transported to one or more mass analyzers that separate the ions based on their mass-to-charge ratio. The mass analyzers can include one or more quadrupolar analyzers, ion traps, time-of flight, magnetic, electromagnetic, ion cyclotron resonance or Fourier transform mass analyzers. The separated ions are detected by one or more detectors that provide data that is used to construct a mass spectrum of the sample.
The ions can be guided, trapped, and analyzed inside a vacuum chamber by devices such as multipole ion guides or linear or 3D-ion traps. For example, in multipole rod assemblies, such as quadrupole, hexapole, octapole or greater assemblies including four, six, eight, or more multipole rods, respectively, the multipole rods are arranged to define an interior volume, in which multipole electric potentials can be generated by applying an oscillating radio frequency (“RF”) voltage on the multipole rods. The multipole electric potentials can guide or trap in the interior volume ions that have mass-to-charge ratios within a specific range that can be selected by the applied RF voltage and, optionally, with a DC bias applied to the multipole rods in the assembly.
Accuracy of the measured mass spectra depends, among other things, on how accurately the actual voltages of the multipole rods can be controlled during operation. To provide such control, the actual voltages can be measured by detector capacitors coupled to the multipole rods. The detector capacitors can provide feedback to a control circuit controlling the RF or DC voltages that are applied to the rods. Based on the feedback, the control circuit can ensure that the actual voltages applied to the multipole rods during operation match the desired RF or DC voltages.
To increase thermal stability, detector capacitors are typically made from materials of low thermal expansion coefficient, such as invar (composition 36% Ni balance iron), quartz or alumina. In addition, parts of the detector can be designed such that any dimensional changes that result from thermal expansion or contraction act to cancel each other to maintain the desired capacitance.
Typically, detector capacitors operate at atmospheric pressure. In one device, a detector capacitor is mounted on a wall that separates a low pressure region inside a vacuum chamber from the atmospheric pressure outside the chamber. In such detectors, the capacitor's gap is inside the low pressure region, and one of the capacitor plates separates the low pressure inside the vacuum chamber from the atmospheric pressure. Such detectors are discussed in more detail in U.S. Pat. No. 6,424,515 to Gore et al., the entire disclosure of which is incorporated by reference herein in its entirety.